1. Field of the Invention
The present invention relates to the production of time variant modifications to the musical waveshape produced in a musical tone synthesizer.
2. Description of the Prior Art
A feature commonly used in electronic organs designed for popular music is the so-called "percussive voice." This is a composite voice including a first tone having a piano-like attack/release envelope played in combination with an organ-like sustained tone. The effect is a transient percussive sound at the onset of tonal production.
A well-known phenomena occurs in pipe organs and is called "chiff." In a wind-blown organ pipe a frequency instability occurs during the build-up of the tone. The instability is approximated by a predominant sound component at the third or fifth harmonic of the pipe's fundamental frequency. This predominant harmonic is transient and quickly diminishes in relative intensity as the nominal fundamental frequency for the pipe begins to "speak" distinctly.
Electronic organs imitate chiff by playing a short grace note at the onset of tone production. The grace note is generated by actuating a 2 2/3-foot coupler for the duration of the nominal attack time for the 8-foot tone which is to be chiffed. Customarily, chiff is used only on diapason and flute tones which are characterized by a spectral composition of very few harmonic components.
The inventor's U.S. Pat. No. 3,740,450 discloses apparatus for producing chiff tones in a digital organ of the type wherein a musical waveshape in repetitively read out from storage at a rate related to the selected note. A chiffing waveshape is stored in a separate memory that is accessed during the attack portion of the primary tone. The separate waveshape memory outputs are combined to produce the chiffed musical tone.
The inventor's U.S. Pat. No. 3,913,442 discloses apparatus for producing chiff tones in an organ of the type described as a Computor Organ in U.S. Pat. No. 3,809,786. In such an instrument, musical tones are produced by computing in real time the amplitudes X(gR) at successive sample points gR of a musical waveshape, and converting these amplitudes to tones as the computations are carried out. The tonal quality of the generated note is established by a set of harmonic coefficients. Transient voices are obtained by adding selected harmonics to the set of harmonic coefficients for a short interval at the onset of the generated waveshape.
One object of the present invention is to implement the production of such transient voices in a polyphonic tone synthesizer. Another object is to produce chiff in a polyphonic tone synthesizer.